backtrace.rs.html -- source
Description: Source to the Rust file `src/libstd/sys_common/backtrace.rs`.
Keywords: rust, rustlang, rust-lang

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    /// Common code for printing the backtrace in the same way across the different
    /// supported platforms.

    use crate::env;
    use crate::io::prelude::*;
    use crate::io;
    use crate::path::{self, Path};
    use crate::ptr;
    use crate::str;
    use crate::sync::atomic::{self, Ordering};
    use crate::sys::mutex::Mutex;

    use rustc_demangle::demangle;

    pub use crate::sys::backtrace::{
        unwind_backtrace,
        resolve_symname,
        foreach_symbol_fileline,
        BacktraceContext
    };

    #[cfg(target_pointer_width = "64")]
    pub const HEX_WIDTH: usize = 18;

    #[cfg(target_pointer_width = "32")]
    pub const HEX_WIDTH: usize = 10;

    /// Represents an item in the backtrace list. See `unwind_backtrace` for how
    /// it is created.
    #[derive(Debug, Copy, Clone)]
    pub struct Frame {
        /// Exact address of the call that failed.
        pub exact_position: *const u8,
        /// Address of the enclosing function.
        pub symbol_addr: *const u8,
        /// Which inlined function is this frame referring to
        pub inline_context: u32,
    }

    /// Max number of frames to print.
    const MAX_NB_FRAMES: usize = 100;

    /// Prints the current backtrace.
    pub fn print(w: &mut dyn Write, format: PrintFormat) -> io::Result<()> {
        static LOCK: Mutex = Mutex::new();

        // There are issues currently linking libbacktrace into tests, and in
        // general during libstd's own unit tests we're not testing this path. In
        // test mode immediately return here to optimize away any references to the
        // libbacktrace symbols
        if cfg!(test) {
            return Ok(())
        }

        // Use a lock to prevent mixed output in multithreading context.
        // Some platforms also requires it, like `SymFromAddr` on Windows.
        unsafe {
            LOCK.lock();
            let res = _print(w, format);
            LOCK.unlock();
            res
        }
    }

    fn _print(w: &mut dyn Write, format: PrintFormat) -> io::Result<()> {
        let mut frames = [Frame {
            exact_position: ptr::null(),
            symbol_addr: ptr::null(),
            inline_context: 0,
        }; MAX_NB_FRAMES];
        let (nb_frames, context) = unwind_backtrace(&mut frames)?;
        let (skipped_before, skipped_after) =
            filter_frames(&frames[..nb_frames], format, &context);
        if skipped_before + skipped_after > 0 {
            writeln!(w, "note: Some details are omitted, \
                         run with `RUST_BACKTRACE=full` for a verbose backtrace.")?;
        }
        writeln!(w, "stack backtrace:")?;

        let filtered_frames = &frames[..nb_frames - skipped_after];
        for (index, frame) in filtered_frames.iter().skip(skipped_before).enumerate() {
            resolve_symname(*frame, |symname| {
                output(w, index, *frame, symname, format)
            }, &context)?;
            let has_more_filenames = foreach_symbol_fileline(*frame, |file, line| {
                output_fileline(w, file, line, format)
            }, &context)?;
            if has_more_filenames {
                w.write_all(b" <... and possibly more>")?;
            }
        }

        Ok(())
    }

    /// Returns a number of frames to remove at the beginning and at the end of the
    /// backtrace, according to the backtrace format.
    fn filter_frames(frames: &[Frame],
                     format: PrintFormat,
                     context: &BacktraceContext) -> (usize, usize)
    {
        if format == PrintFormat::Full {
            return (0, 0);
        }

        let skipped_before = 0;

        let skipped_after = frames.len() - frames.iter().position(|frame| {
            let mut is_marker = false;
            let _ = resolve_symname(*frame, |symname| {
                if let Some(mangled_symbol_name) = symname {
                    // Use grep to find the concerned functions
                    if mangled_symbol_name.contains("__rust_begin_short_backtrace") {
                        is_marker = true;
                    }
                }
                Ok(())
            }, context);
            is_marker
        }).unwrap_or(frames.len());

        if skipped_before + skipped_after >= frames.len() {
            // Avoid showing completely empty backtraces
            return (0, 0);
        }

        (skipped_before, skipped_after)
    }

    /// Fixed frame used to clean the backtrace with `RUST_BACKTRACE=1`.
    #[inline(never)]
    pub fn __rust_begin_short_backtrace<F, T>(f: F) -> T
        where F: FnOnce() -> T, F: Send, T: Send
    {
        f()
    }

    /// Controls how the backtrace should be formatted.
    #[derive(Debug, Copy, Clone, Eq, PartialEq)]
    pub enum PrintFormat {
        /// Show only relevant data from the backtrace.
        Short = 2,
        /// Show all the frames with absolute path for files.
        Full = 3,
    }

    // For now logging is turned off by default, and this function checks to see
    // whether the magical environment variable is present to see if it's turned on.
    pub fn log_enabled() -> Option<PrintFormat> {
        static ENABLED: atomic::AtomicIsize = atomic::AtomicIsize::new(0);
        match ENABLED.load(Ordering::SeqCst) {
            0 => {}
            1 => return None,
            2 => return Some(PrintFormat::Short),
            _ => return Some(PrintFormat::Full),
        }

        let val = env::var_os("RUST_BACKTRACE").and_then(|x|
            if &x == "0" {
                None
            } else if &x == "full" {
                Some(PrintFormat::Full)
            } else {
                Some(PrintFormat::Short)
            }
        );
        ENABLED.store(match val {
            Some(v) => v as isize,
            None => 1,
        }, Ordering::SeqCst);
        val
    }

    /// Prints the symbol of the backtrace frame.
    ///
    /// These output functions should now be used everywhere to ensure consistency.
    /// You may want to also use `output_fileline`.
    fn output(w: &mut dyn Write, idx: usize, frame: Frame,
                  s: Option<&str>, format: PrintFormat) -> io::Result<()> {
        // Remove the `17: 0x0 - <unknown>` line.
        if format == PrintFormat::Short && frame.exact_position == ptr::null() {
            return Ok(());
        }
        match format {
            PrintFormat::Full => write!(w,
                                        "  {:2}: {:2$?} - ",
                                        idx,
                                        frame.exact_position,
                                        HEX_WIDTH)?,
            PrintFormat::Short => write!(w, "  {:2}: ", idx)?,
        }
        match s {
            Some(string) => {
                let symbol = demangle(string);
                match format {
                    PrintFormat::Full => write!(w, "{}", symbol)?,
                    // strip the trailing hash if short mode
                    PrintFormat::Short => write!(w, "{:#}", symbol)?,
                }
            }
            None => w.write_all(b"<unknown>")?,
        }
        w.write_all(b"\n")
    }

    /// Prints the filename and line number of the backtrace frame.
    ///
    /// See also `output`.
    #[allow(dead_code)]
    fn output_fileline(w: &mut dyn Write,
                       file: &[u8],
                       line: u32,
                       format: PrintFormat) -> io::Result<()> {
        // prior line: "  ##: {:2$} - func"
        w.write_all(b"")?;
        match format {
            PrintFormat::Full => write!(w,
                                        "           {:1$}",
                                        "",
                                        HEX_WIDTH)?,
            PrintFormat::Short => write!(w, "           ")?,
        }

        let file = str::from_utf8(file).unwrap_or("<unknown>");
        let file_path = Path::new(file);
        let mut already_printed = false;
        if format == PrintFormat::Short && file_path.is_absolute() {
            if let Ok(cwd) = env::current_dir() {
                if let Ok(stripped) = file_path.strip_prefix(&cwd) {
                    if let Some(s) = stripped.to_str() {
                        write!(w, "  at .{}{}:{}", path::MAIN_SEPARATOR, s, line)?;
                        already_printed = true;
                    }
                }
            }
        }
        if !already_printed {
            write!(w, "  at {}:{}", file, line)?;
        }

        w.write_all(b"\n")
    }

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